This invention relates to a single crystal substrate of an electrically insulating material for epitaxial growth thereon of a semiconductor layer. A substrate according to this invention is therefore for use in manufacturing semiconductor devices.
A substrate of an electrically insulating material is theoretically welcomed for epitaxial growth thereon of a semiconductor layer. This is because the substrate affords excellent electric insulation between the device elements of an integrated circuit formed by the use of the semiconductor layer and eliminates the undesired parastic capacitance. A sapphire (Al.sub.2 O.sub.3) or a spinel (mgAl.sub.2 O.sub.3) single crystal has been used as a substrate of the type described. There is, however, a considerable lattice mismatch between the substrate single crystal and the epitaxially grown semiconductor single crystal. The mismatch results in stress latent in the semiconductor layer and dislocation in the semiconductor single crystal to render the semiconductor crystal imperfect and heterogenaous and to adversely affect the electrical characteristics of the semiconductor layer, such as mobility and lifetime of charge carriers, and of the semiconductor devices manufactured by the use of the semiconductor layer.
In Japanese Patent Application No. Syo 47-74483 filed July 25, 1972, and laid open to public on Mar. 28, 1974, Jun-ichi Nishizawa, one of the present inventors, has taught a combination of a substrate of the type described and a semiconductor layer epitaxially grown thereon. The lattice mismatch in question is reduced in general in accordance with Nishizawa by changing the composition of at least one of the substrate and the layer of the combination. In the meantime, Jun-ichi Nishizawa and Mitsuhiro Kimura, the instant inventors, have disclosed in U.S. patent application Ser. No. 600,007 filed July 29, 1975 now U.S. Pat. No. 3,990,902, a spinel single crystal substrate of the type described wherein the mismatch is reduced more specifically.
A preferred substrate, however, has not been known, which consists of a sapphire containing single crystal and is for use in epitaxially growing thereon a semiconductor layer with the lattice mismatch reduced to an admissible extent.